The present invention relates generally to systems and methods for performing noninvasive surgical procedures using focused ultrasound, and more particularly to systems and methods for reducing secondary hot spots created by a focused ultrasound transducer array.
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby obviating the need for invasive surgery. For this purpose, piezoelectric transducers driven by electric signals to produce ultrasonic energy have been suggested that may be placed external to the patient but in close proximity to the tissue to be ablated. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused at a xe2x80x9cfocal zonexe2x80x9d corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of sonications is used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array, such as that disclosed in U.S. Pat. No. 4,865,042 issued to Umemura et al., has been suggested for this purpose. This spherical cap transducer array includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings generally have equal surface areas and may also be divided circumferentially into a plurality of curved transducer elements or sectors, creating a sector-vortex array. The transducer elements are driven by radio frequency (RF) electrical signals at a single frequency offset in phase and amplitude. In particular, the phase and amplitude of the respective drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired xe2x80x9cfocal distance,xe2x80x9d i.e., the distance from the transducer to the center of the focal zone and provide a desired energy level in the target tissue region.
Although the transducer elements are focused at a desired primary focal zone, there may also be one or more secondary focal zones at locations other than the intended primary focal zone. For example, spaces between the concentric rings may contribute to such xe2x80x9chot spots,xe2x80x9d particularly in the xe2x80x9cnear field,xe2x80x9d i.e., the region between the transducer and the primary focal zone. Such secondary hot spots may lead to undesired heating, pain for the patient, and/or possibly necrosis of tissue in the near field. Because the transducer is made up of a finite number of rings, the step function used to change the phase between the rings may also contribute to the creation of secondary hot spots.
To minimize the effects of secondary hot spots, one proposed solution has been to use a wide-band frequency signal to drive the transducer elements, such that the location of the secondary hot spots may be xe2x80x9csmearedxe2x80x9d by this wide-band signal, i.e., the energy diffused within tissue regions at different locations within the patient, thereby reducing the risk of heating the tissue regions sufficiently to necrose them. This solution, however, may be limited by the transducer bandwidth and may require special complicated electronics. Wide-band drive signals may also smear the primary focal zone, thereby requiring increased ultrasonic energy delivery to ablate the target tissue at the primary focal zone. The smearing of the primary focal zone may be at least partially corrected by introducing appropriate delays in the signals, but this may substantially complicate control of the phased array, for example, requiring additional electronic phasing and focusing.
Accordingly, it would be desirable to provide systems and methods for treating a tissue region using ultrasound energy at one or several discrete frequencies that reduces secondary hot spots, without substantially reducing the intensity at the Be primary focal zone.
The present invention is directed to systems and methods for performing a therapeutic procedure using focused ultrasound that substantially minimizes the effects of secondary hot spots, without adversely impacting on the energy delivered to the primary focal zone.
In a preferred embodiment, a focused ultrasound system includes a transducer formed from piezoelectric material that includes a plurality of transducer elements. The transducer elements may be provided in a variety of arrays or geometries. For example, in one exemplary embodiment, the transducer may be a substantially concave phased array, including a plurality of concentric rings. Each transducer ring may be divided circumferentially into a plurality of curved elements or xe2x80x9csectors.xe2x80x9d Alternatively, a linear array of transducer elements may be provided. Other arrangements or geometries of transducer elements may also be provided, such as a checkerboard pattern, a hexagonal lattice, or a random pattern of transducer elements, and the invention should not be limited to any one particular geometry.
Drive circuitry is coupled to each of the respective transducer elements for providing drive signals to each transducer element at one of a plurality of discrete frequencies, preferably at radio frequencies. A controller is coupled to the drive circuitry for periodically changing a frequency of the drive signals during a single sonication between one of the plurality of discrete frequencies. In particular, the controller determines a phase component for each of the respective drive signals provided, such that a primary focal zone of a given size and shape results at a predetermined distance from the transducer.
The transducer is preferably adjustably mounted within a casing, such as a fluid-filled table, onto which a patient may be disposed. During use of the system, the plurality of transducer elements may be activated with a set of drive signals, each at a single frequency, while focusing ultrasonic energy produced by the transducer elements at the primary focal zone, corresponding to the location of a target tissue region in a patient. Periodically, the frequency of the respective drive signals may be changed, while substantially maintaining the focus at the primary focal zone. Preferably, this is achieved by controlling the phase component of the drive signals when the frequency of the drive signals is changed.
The series of drive signals is provided to the transducer to create a single sonication that substantially ablates the tissue at the target tissue region, while minimizing the effects of secondary focal zones or xe2x80x9chot spots.xe2x80x9d As the frequency is changed, the location of the secondary hot spots may change, thereby dispersing the ultrasonic energy at the secondary hot spots to several locations within the patient""s body. Thus, while the target tissue region receives sufficient energy to substantially necrose the tissue there, the tissues at the secondary hot spots do not.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.